The present invention relates to dc electric motor current regulators and, more particularly, to a method and apparatus for regulating electrical braking current in a dc electric traction motor.
In present day electric vehicles, electronic power regulators are used to control the torque, or speed, developed by the electric traction motors. Typically, the regulator comprises a time-ratio or chopper circuit which varies the power developed by the motors by controlling the percentage of time that the motors are connected directly to a power source. For maximum mobility, the power source is a battery, which limits the available power to the motors. The regulator also includes apparatus responsive to accelerator position for varying the mark-space ratio of the chopper circuit.
In order to reduce the rate of wear of mechanical brakes in electric vehicles such as forklifts or other industrial trucks, it is common practice to implement some form of electrical braking. A common form of electrical braking is dynamic braking or plugging in which the motor armature is short-circuted by a diode and motor field current, and therefore, torque is regulated by the chopper circuit. In general, low level of field current generates a relatively high magnitude of armature current. Regulation to a desired braking torque under these conditions tends to be inefficient since the armature current is so large with respect to field current that armature reaction disturbs the normal field flux control of armature current. Because such a low level of field current excitation at higher armature velocities produces very large magnitudes of armature current, the control of initiation of plugging becomes relatively critical.
When a chopper circuit is used to control motor current, the transition from a motoring mode to a braking mode typically requires that the chopper circuit switch from a relatively high percent on-time to a relatively low percent on-time. A typical transition might be from a 95 percent on-time per cycle to a one percent on-time per cycle. The low mark-space ratio at plug initialization sometimes caused cogging or jerking due to double pulsing, a condition resulting from one current pulse bringing the plug current almost to a desired level followed by a second pulse forcing a large current overshoot. Part of the reason for double pulsing comes from the fact that the control loops are slightly unstable due to a large time delay between armature current and machine flux. The chopper applies voltage to the field winding while armature current is being monitored. By the time armature current reaches the desired value, field current, and therefore machine flux, has increased above the level necessary to generate the desired armature current thus causing a torque overshoot. To some extent, the plug current regulating problem stemmed from the need to be able to use the same current sensor in both motoring and braking modes of operation. The typical current sensor filtering to provide an indication of average current and in electric vehicle applications might respond to a point at about 80 percent up on the ripple curve, i.e., the typical approach is to attempt to regulate the current peaks. In these applications, transient voltage spikes resulting from the chopping action in the inductive circuit have high values which affect the accuracy of the control system.
It is an object of the present invention to provide an improved current regulating system for electrical brakes of a dc electric motor.
It is another object of the invention to provide a plug current regulating system for a dc electric motor switching regulator control system which reduces jerking torque on the motor.
It is another object of the invention to provide a plug current regulating system for a dc electric motor switching regulator control system which avoids double or multiple pulsing of the motor.
It is a further object of the invention to provide a plug current regulating system for a dc electric motor switching regulator control system which responds to absolute values of armature current without filtering.